Sr isotopes in metasomatized fault rocks constrain the age of juxtaposition of oceanic and continental lithosphere (Dun Mountain Ophiolite, New Zealand)

The juxtaposition of oceanic and continental lithosphere along major fault zones is a rare but globally significant process that is commonly associated with the closure of an ocean basin. However, it is often difficult to determine the timing in the absence of a high-grade metamorphic sole that can be dated. We show that the timing of mantle and crust juxtaposition along the base of the Dun Mountain Ophiolite in New Zealand can be constrained using Sr isotope data from metasomatic rocks that developed due to the extreme chemical contrast between ultramafic (mantle) and quartzofeldspathic (crustal) lithologies along the basal Livingstone Fault. Fluids that migrated through ultramafic rocks into greenschist facies quartzofeldspathic metasediments resulted in formation of actinolite, diopside, hedenbergite, epidote and garnet, as well as the disappearance of quartz and white mica, and the near-complete removal of Rb, in a rodingite and albite-rich zone up to c.10 m wide. Major and trace element compositions indicate that albite-rich rocks and rodingites have quartzofeldspathic protoliths equivalent to rocks outside of the metasomatic zone. Due to loss of the main Rb-bearing phase (mica), the timing of metasomatism can be broadly constrained by age correction of the 87Sr/86Sr ratios of unaltered quartzofeldspathic rocks to those of the strongly metasomatized equivalents. The calculated age, c. 173 Ma, represents juxtaposition of continental and oceanic lithosphere along the Livingstone Fault. Although obduction of the Dun Mountain Ophiolite may have initiated prior to the dated event, the new age is consistent with published paleogeographic reconstructions, and indicates that a >2000 km long fore-arc ocean basin that existed along the Gondwana margin was closed by the middle Jurassic. Removal of Rb from metasedimentary rocks in contact with ultramafic rocks in ophiolite settings provides an opportunity to constrain the timing of juxtaposition at lower-greenschist facies conditions.